Railway vehicles are provided with traction motors and gearboxes mounted on their bogie frames, and they run on rails on power transmitted from the traction motors to wheel sets through the gearboxes.
FIG. 1 is a plan view of a conventional bogie frame. FIG. 2 is an enlarged plan view showing a region where a mounting bracket is located in a cross frame of the conventional bogie frame. As shown in FIG. 1, the bogie frame 101 includes a pair of left and right side frames 102 disposed along a direction of rails and cross frames 103 that connect the side frames 102 to each other. The cross frames 103 are each composed of a cylindrical steel pipe, and are welded at both ends to the side frames 102.
A mounting bracket 104 for mounting a traction motor (hereinafter also referred to as “mounting bracket 104 for a traction motor”) is welded to each cross frame 103 at its outer peripheral surface, and further a mounting bracket 105 for mounting a gearbox (hereinafter also referred to as “mounting bracket 105 for a gearbox”) is welded to each cross frame 103 at its outer peripheral surface (see, for example, Patent Literatures 1 and 2).
It is to be noted that the mounting bracket 104 for a traction motor and the mounting bracket 105 for a gearbox are components that are totally independent and separate from the cross frame 103 before being joined thereto. The mounting bracket 104 for a traction motor is formed with a rib structure in many cases because it is subjected to a heavy load such as the reaction force of power from the traction motor and the weight of the traction motor itself during travel of the railway vehicle. The same applies to the mounting bracket 105 for a gearbox. As a result, the conventional bogie frame 101 has many weld zones having short weld lines because the rib-structured mounting brackets 104 and 105 are welded to the cylindrical cross frames 103. In a weld zone, weld quality tends to deteriorate at start and end points of the weld, and thus zones having short weld lines are more likely to have this tendency.
Furthermore, stress concentration is more likely to occur in the weld zones between the mounting brackets 104 and 105 and the cross frame 103 because of the heavy load applied to the mounting brackets 104 and 105 during travel of the railway vehicle. In particular, as shown in FIG. 2, at the base end regions of the upper surface portions and the lower surface portions of the mounting brackets 104 and 105 that are joined to the cross frame 103 (the regions indicated by bold line circles in FIG. 2), there exist toes of fillet welds which are referred to as “fillet weld toes”, and stresses are concentrated at the fillet weld toes. Because of this, it is essential that the fillet weld toes be treated for smoothing with a grinder or the like.
Bogie frames are required to have good welding workability for assembling because the work of assembling mostly involves welding. However, as described above, the conventional bogie frame 101 is provided with many weld zones having short weld lines and in addition with fillet weld toes that are required to be treated, when the mounting brackets 104 for a traction motor and the mounting brackets 105 for a gearbox are welded to the cross frames 103. Thus, the conventional bogie frame 101 is more likely to suffer deterioration of the weld quality at the weld zones, and has low efficiency for the work of welding. Therefore, the conventional bogie frame 101 is provided with poor welding workability for assembling.